Tree sap is a general term for the fluids circulating within a tree’s vascular system, similar to the circulatory system in animals. This fluid movement is fundamental to a tree’s survival, distributing necessary materials for growth and energy production. The term “sap” covers two chemically distinct fluids, each with its own specialized transport system. These internal fluids carry the water, dissolved minerals, and sugars that sustain the entire organism.
The Two Transport Systems
Trees utilize two specialized systems, the xylem and the phloem, to move fluids throughout their structure. The xylem system is composed of dead, hollow cells that form continuous, tube-like pathways. Xylem is primarily responsible for transporting water and dissolved inorganic nutrients upward from the roots. The phloem system consists of living cells, such as sieve-tube elements, and its main role is translocation. This process moves organic compounds from where they are produced (the source) to where they are needed (the sink), typically in a bi-directional flow for flexible energy distribution.
What Xylem Sap Contains
Xylem sap is a dilute, watery solution, often consisting of over 99% water by volume. Its primary contents are inorganic mineral ions absorbed from the soil, such as nitrogen, potassium, and calcium. The function of this fluid is to replace water lost through transpiration and to supply necessary building blocks for new cell growth. While mostly water and minerals, xylem sap also contains trace amounts of organic compounds. These include plant hormones, which regulate growth, and small quantities of amino acids, though concentrations vary depending on the season.
What Phloem Sap Contains
Phloem sap is significantly more complex and viscous than xylem sap because its main role is energy transport. The predominant component, aside from water, is the disaccharide sugar sucrose, the form of energy produced during photosynthesis. Sucrose typically makes up a high concentration (5% to 10%), creating the osmotic pressure necessary to drive flow. The fluid is also rich in nitrogenous compounds, primarily amino acids, which are transported to growing tissues. Additionally, phloem sap contains proteins, hormones, and signaling molecules that coordinate the tree’s response to environmental cues, and this concentrated sugar solution is harvested to create maple syrup.
Sap Versus Resin and Pitch
A common point of confusion is the difference between true sap, the internal transport fluid, and external secretions like resin or pitch. Resin is a thick, sticky, aromatic substance that is chemically distinct from sap. It is not part of the tree’s circulatory system but is a secondary metabolite secreted within specialized resin canals. Resin is primarily a defensive compound, acting like a protective scab when the tree is wounded. Its composition seals the injury, deters pests, and prevents infection, and while all trees produce sap, true resin is most characteristic of coniferous trees.